Process for the liquefaction and reliquefaction of natural gas



Nov. 8, 1960 Filed Jan. 29, 1958 H. KNAPP 2,959,020

PROCESS FOR THE LIQUEFACTION AND RELIQUEFACTION OF NATURAL GAS 2 Sheets-Sheet 1 "1 INVENTOR.

BY fielmui Knapp a PROCESS FOR THE LIQUEFACTION AND RELIQUEFACTION OF NATURAL GAS Filed Jan. 29, 1958 H. KNAPP Nov. 8, 1960 2 Sheets-$heet 2 mqwmini I LHF INVENTOR. flelmuz Knapp BY wiwwm ml United States Patent PROCESS FOR THE LEQUEFACTION AND RELIQUEFACTIGN F NATURAL GAS Helmut Knapp, Riverdaie, N.Y., assignor, by mesne assignments, to Conch International Methane Limited, a corporation of the Bahamas Filed Jan. 29, 1958, Ser. No. 712,002

5 Claims. (Cl. 62-9) This invention relates to the liquefaction of natural gas and it relates more particularly to the reliquefaction of vapors given ofi by liquefied gas in storage or in transportation.

The invention will hereinafter be described with reference to the liquefaction of a natural gas composed mostly of methane but it will be understood that the concepts described and claimed will be applicable to the liquefaction or reliquefaction of other low boiling gases such as hydrogen, helium, oxygen, nitrogen, air and the like.

Natural gas has found extentive use both in the United States and in countries foreign to the United States as a fuel for space heating and for the generation of power or as a raw material used for synthesis in the petro-chemical industry. It is well known that certain areas of the country and that certain portions of the world are fortunate in having ample supplies of natural gas while other areas of the country and many countries of the world have no natural gas or an insufificient supply of natural gas.

Where the source of plentiful supply is not separated by a large body of water from the areas where a deficiency exists, use can be made of pipe lines for the transportation of the natural gas while in a gaseous state. Where, however, a large body of water separates the source of supply from the areas where a deficiency exists or when the later area is isolated, a pipe line as a means for transportation of the natural gas is not available and other means must be employed, if such means can be made economically available.

The use of containers for transportation of the natural gas while in a gaseous state is impractical because of the relatively small amount of natural gas that can be carried in the space available in the containers constructed and in ships or other transportation means employed for the containers. It is known, however, that about 600 times the amount can be carried per unit space when the natural gas is in the liquefied state as compared to the gaseous state at equivalent pressure. Thus it is a concept of this invention to reduce the natural gas to a liquefied state at the source of plentiful supply for transportation of the liquefied natural gas in suitable insulated containers either by ship or other conveyance means to the area where deficiencies exist and where the liquefied gas can be reformed to its gaseous state for use.

' For economical and practical operation, it is desirable to transport the liquefied natural gas in large volume. Because of the expense in the construction of containers of large volume for operation under high positive pressure, it is desirable to effect the transportation and storage of the cold boiling liquefied gas while at atmospheric pressure or preferably slightly above atmospheric pressure to prevent leakage of air into the storage vessel.

Liquefied natural gas at about atmospheric pressure has a boiling point temperature in the order of about 250 F. to about 258" F., depending upon the amount of heavier hydrocarbons, such as ethane, propane, butane and the like which may be present in the liquefied mateice rial. Thus it is essential to make use of containers which are highly insulated to minimize the leakage of heat from the ambient atmosphere into the liquid to cause vaporization. While the use of insulated tanks will minimize the amount of liquid lost by vaporization, some heat flow from the ambient atmosphere to the liquefied natural gas will naturally take place because of the large temperature differential. As a result, some of the cargo of liquefied natural gas in storage will be released as a vapor. For the most part, this vapor will be composed of methane because of its lower boiling point by comparison with the other heavier hydrocarbons which might be present and it will be assumed, for the purposes of this disclosure, that the vapors coming off of the storage vessels are relatively pure methane. Tosome extent, the released methane vapors can be used as a fuel to power the ship or other transportation means during travel from the source of supply to the area of use. It is sometimes desirable, however, to make use of other liquid fuels which can be carried in limited amounts in available space on the ship and/or to recover the methane vapors for return in a liquefied state to the storage means. Means such as described in the copending application Ser. No. 698,667, filed on November 25, 1957, might be employed but the compressors and heat exchangers required for a modified cascade cycle of the type described will occupy too much space for use on a ship.

Thus it is an object of this invention to provide a liquefaction system for gaseous materials and it is a related object to provide a method and apparatus for the liquefaction or reliquefaction of natural gas.

More particularly, it is an object of this invention to provide a liquefaction system which is applicable generally to the liquefaction of natural and other gases; which is applicable to the reliquefaction of gaseous vapors given off during storage and transportation of a liquified low boiling gas; which is simple in construction and eco nomical and efficient in operation; which occupies minimum space from the standpoint of equipment to enable use in the limited space available on ship for liquefaction; which requires a minimum amount of equipment for operation and which can make use of the vapors available to power the equipment for liquefaction.

These and other objects and advantages of this invention will hereinafter appear and for purposes of illustration, but not of limitation, an embodiment of this invention is shown in the accompanying drawing in which- Figlre 1 is a flow sheet illustrating the practice of this invention, and

Figure 2 is a flow sheet embodying a modification in the process illustrated in Figure 1.

While description will be made to the reliquefaction.

can be used as a means for liquefying natural gas transported by pipe line to put into practice the process of peak shaving whereby an amount of natural gas can be liquefied for storage when the supply exceeds the demand to make gas available for use when demand is at its peak and would otherwise exceed supply. This enables the peaks to be shaved down so as to provide for operation of the pipe line at maximum capacity for most economical transportation of the natural gas.

In accordance with the practice of this invention, liquefaction of the methane is achieved by the condensation of the methane while at high pressure whereafter the condensed methane is let down to the pressure desired for storage. The amount of refrigeration available from the relatively small amount of cold gas recycled from storage is insufficient to reduce the temperature of the compressed gas whereby most of the processed gas will be in the condensedstate upon return to storage. The desired results are achieved-"in a substantially self-contained process by using, as a refrigeration medium, a portion of the gas which has been compressed and expanded with external work andrecycled to the main gas stream after use of the expanded gas, in heat exchange relation with the main gas stream at high pressure to achieve liquefaction.

When, as is preferred, storage and transportation of the liquefied natural gas is carried out in insulated containers maintained, for example, at about atmospheric pressure or slightly above, the vapors released from the container will have a temperature of about -245' F.i10 F.

In reliquefaction, the natural gas from the containers 10. is caused to flow through line 12 into a series of heat exchangers, as illustrated by the heat exchangers 14 and 1 6, to recover refrigeration available from the vapors. In the illustrated modification for the reliquefaction of vapors released from storage, the temperature of the vapor. is raised to. about +33- F. upon passage through the first heat exchanger 14 and to about 85 F. upon passage through the second heat exchanger 16; The vapors can be processed through compressors without raising lubrication problems. It is preferred to increase the pressure of the vapor by multi-stage compression stepseach of which is preferably followed by a cooling step to remove heat of compression whereby a highly compressed gas is delivered at ordinary temperature for liquefaction by condensation. Isentropic compression or compression at constant entropy is preferably achieved by a reciprocating compressor of conventional construction as distinguished from a centrifugal compressor because the former is more eflicient when a lower volume of gas is being processed. It will be understood, however, that centrifugal compression could be employed.

In the illustrated modification compression is achieved in four stages to raise the pressure of the gas orvapor from slightly above atmospheric pressure to about 2000:1000 p.s.i. In the first stage, the reciprocating compressor 20 receives the heated up vapor from line 18 and delivers the vapor to line 22 at about 50-60 p.s.i. and about 260 F. Heat of compression is removed, as by a water cooler 24 which reduces the compressed gas to a temperature of about 100 F; for delivery to line 26. In the second stage of compression, the compressor 28 raises the gas to a level of about 180-190 p.s.i. with consequent increase in temperature to about 260 F. Heat of compression is again removed by passage of the compressed gas through a subsequent water cooler 30. to reduce the temperature to about 100 F. In the third stage, the compressor 32; raises the gas to a pressure of about 6l06 25 p.s.i. and;

to a temperature of about 260 F. and the water cooler 34 takes the temperature back down to about100 F;

In the fourth and final stage, the compressor 36 raises the gas to a pressure of about 2000' p.s.i. andto a temperature of about 290 F. and the after cooler 38, in

the form of a water cooler, removes heat ofcompression to bring the temperature back down to abouti100?*F.-

It will be understood that the compression stage s represented may beembodied as separate stages in a single compressor, as illustrated in Figure 2, or various conibinations of compressors, and. that other coolers, such as an air cooler, can be used as the after coolers to take out heat of compression andto reduce the temperature of the compressed gases.

the subsequent equipment by condensing and plating out on the walls of the heat exchangers when reduced to the low temperature levels to which the natural gas stream is subsequently processed. For this purpose, the recornpressed gas is processed through a separator 40 after the temperature of the compressed gas has been reduced by the final after cooler 38 to a temperature above the condensation temperature of the gas but below the condensation temperature of the lubricant vapors.

The clean and compressed gas is then advanced from the separator 40 through line 42 to the heat exchanger 16 and from the heat exchanger 16 the main process stream is advanced through line 44 to the heat exchanger 14 to recover some of the refrigeration available in the cold gas recycled from storage thereby to make use of the refrigeration available for reducing the temperature of the compressed gas.

The main process stream in line 46, which is intended to be in a liquefied state, is expanded at constant en thalpy through an expansion valve 48 to drop the liquid from a, pressure of 2000 p.s.i. to a pressure of about- 60 p.s.i. whereby some of the liquid is flashed off with a slight reduction in temperature. The wet gas is fedthrough line 50 into a separating drum 52' wherein the condensate is separated from the gaseous component. The condensate is drained off through line 54 and flashed through valve 56 to the pressure conditions existing in the storage container 10 for return of the flashed condensate to the container for storage. The gas flashed oif from the expansion of the condensate is joined with the boil-ofi from the heat leaks into storage and returned through line 12' to form a part of the processing stream.

It, has been found that the amount of cold available from the small amount of gas which is vaporized from the material in storage due to heat leak and from the final pressure reduction is insuflicient to reduce the temperature of the main process stream to a level Where an adequate amount of the gas in the main process stream remains in the condensed state upon subsequent reductions in pressure. This deficiency has, been overcome, in accordance with the practice of this invention, by modification in the process to make use of a part of the process stream to supply a large amount of cold gas for use in heat exchange relation with the main process stream to reduce the temperature of the stream to a level whereby the gas is condensed before. reduction in pressure and only a small proportion of the condensed gas is flashed off during said pressure reduction step or steps.

Thus, in accordance with an important concept of this invention, a lot of cold gas is made available for refrigeration, by taking ofl" an increment of thecompressed gas from the main process stream into line 60for isentropic expansionby a machine doing external work froma pressure of about 2000 p.s.i. to about 60-p;s.i. or to a level corresponding to an intermediate stage ofrcompression, such as after-the first stage, so that-the ex panded gas. can be led back interstage into the recompression cycle. Expansion with work can be carried out in a centrifugal'expander but, because of the smallvolume ofmaterial being processed, it is preferredto'make' use of a reciprocating expander 62. Expansion with work provides a large amount of materialin a gaseous state aQabout 2 20'F.since 1ittle, if any, condensation tionwiththe main process stream to condense the main progess,stream,under the temperature andpressure conditions existing. The cold gas at intermediate pressure-- enters the heat exchanger 14 at about 220 F. and leaves at about 33 F. and it leaves the heat exchanger 16 at a temperature of about 85-100 F. Instead of processing the large amount of expanded refrigeration gas bled from the main process stream through the separator 52, the refrigeration gas may by-pass the separator 52 for feeding directly to the heat exchangers 14 and 16, as illustrated by the flow sheet in Figure 2 of the drawing. Any amount of condensate present in the refrigeration gas will become vaporized in the heat exchangers for return as a gas to the compression cycle. It is preferred, however, to recover the condensed portions where economical for return to storage.

When such large amount of cold gas is made available for use in heat exchange relation to extract heat from the main process stream, the main process stream will be capable of being reduced in temperature from 100 F. to about 32 F. upon passage through the heat exchanger 16 and to about 2l5 F. upon passage through the heat exchanger 14 whereby the gas becomes totally condensed. Thus the heat exchanger 14 can be referred to more accurately as a liquefier. When reduced from 2000 p.s.i. to about 60 p.s.i. upon passage through the valve 48, a small amount of the condensed gas will be flashed off as a vapor with a consequent reduction in temperature from --215 F. to a temperature of about --220 F. Under the conditions described less than about 20-25 percent of the condensed gas will be flashed off as a vapor upon pressure drop, the remainder continuing as a condensate for return to storage.

It will be apparent from the foregoing that the large amount of refrigeration gas will be at about the same temperature and pressure as the main gas stream, yet one will be in a gaseous state while the other is in a liquefied state. The difference between the streams whereby the one is condensed to a liquid and the other remains as a gas is in the latent heat which is removed from the main gas stream by heat exchange with the refrigeration gas.

Both the expansion valve 48 and the valve 66 in line 64 are provided with back pressure controllers 68 and 70 respectively to maintatin positive control of the pressure conditions existing in the units. Controller 68 can be eliminated and a floating pressure provided by direct communication interstage of the compressures to return the gas at the interstage compressure for joinder with the main gas stream.

The refrigeration gas and the dry gas component of the expanded main process stream is returned to the cycle interstage between the first and second stages of compression 20 and 28 respectively thereby to avoid the necessity of recompressing the largest portion of the gas through all of the stages. Instead of returning the recycled gas interstage, some or all of the gas may be bled from the system for use as fuel in powering the units employed in the described liquefaction cycle or for powering the conveyance means.

It is conceivable that, at times, the amount of gas available for recycling through the system may exceed the capacity of the system, as when the liquefied natural gas is being loaded into the storage containers and a large amount of boil-off occurs. Under such and related circumstances, a connection 72 can be provided with a valve control 74 for returning surge vapors to shore for use or re-liquefaction.

The pressure and temperature conditions described are merely given by way of illustration of a set of conditions which may be employed. It will be understood that other temperatures and pressures could be used in the practice of this invention. For example, instead of raising the pressure of the gas by four stages of compression to 2000 p.s.i., fewer stages may be used, such as three stages, to raise the pressure of the gas to 600 p.s.i. The difference would result in greater power requirement because of the greater amount of flashing upon corresponding drop in temperature. Similarly, the pressure of the process stream may be raised to 1500 p.s.i. or 2500 p.s.i. with corresponding changes in temperature and pressure during processing for liquefaction and with corresponding changes in the amount of condensate which is derived upon expansion.

In the supply of a large amount of refrigeration gas to supplement the small amount of gas recycled from heat leak into storage, the intent is to process an amount of gas through the expander 62 to supply an additional amount of refrigeration suflicient to reduce the temperature of the main process stream for liquefaction of an amount of vapor equivalent to the methane vapors escaping from storage due to heat leaks. The amount will vary in accordance with the conditions existing. Thus, the concept is to split an increment from the main stream while in a gaseous state wherein balance between temperature and pressure can be controlled to produce a large amount of cold gas for use to liquefy the main portion of the gas stream.

As previously pointed out, instead of cycling the expanded refrigeration stream through the separator 52, it can by-pass the separator for passage directly through the heat exchangers but, in such event, the refrigeration stream might be processed through a separator to remove what little amount of condensate might exist. The main process stream may be reduced in a single step or in multiple steps to the pressure for storage without making use of a separator 52 in the line, but it is preferred to effect reduction in an intermediate low pressure so that the fluids and gases will be under suflicient pressure to flow by their own force for delivery to the subsequent processing steps and equipment. An intermediate pressure other than 60 p.s.i. can be employed. It the pressure is reduced all the way to the pressure of storage, then the flash into storage will be correspondingly increased.

It will be apparent from the foregoing that I have provided a simple and efficient means to achieve reliquefaction of the relatively small amount of vapors which are given off because of heat leaks into storage of a low boiling liquefied gas and it will be evident further that the concepts described for 'reliquefaction of the relatively small amount of vapors may be adapted as a liquefaction process for a main gas stream of materials.

It will be understood that changes may be made in the details of construction, arrangement and operation and in the conditions of operation without departing from the spirit of the invention, especially as defined in the following claims.

I claim:

1. In the process for liquefying natural gas, the steps of supplying the gas continuously as a main process stream at high pressure, continuously removing a portion of the high pressure gas from the main process stream, expanding the removed portion of the gas with work to an intermediate pressure to produce a large volume of cold gas with a minimum amount of condensation, passing the large volume of cold and expanded gas in heat exchange relation with the main process stream to reduce the temperature of the gas in the main process stream to liquefaction temperature, reducing the pressure of the main process stream to an intermediate pressure to provide a condensate portion and a gaseous portion, separating the condensate from the gaseous remainder in the main process stream, flashing the condensate to about atmospheric pressure for removal to storage and passing the gaseous remainder in the main process stream and the gas released upon flashing in heat exchange relation with the main process stream prior to expansion thereof to an intermediate pressure.

2. The process as claimed in claim 1 wherein at least a part of the main stream constitutes natural gas vapors released from liquefied natural gas in storage and wherein the liquefaction process comprises a reliquefaction of su chvaporsiandincludesthe step of recompressing said.

natural gas vapors to the pressure. of the, main stream,

3,. The process as claimed in claim 1 which, includes the step. of continuously returning thegaseous-portio-n removed from the main stream for refrigeration for rejoinder with the main process stream to form a part thereof prior to. the iremovalof a portion thereof asthe refrigeration increment.

' 4, The processes claimed in claim .1 which includes the step of continuously returning the gaseous remainder and the. flashed gas for rejoinderwith thernain process stream .to form a part thereof after said gaseous remainder andthe flashed gas have been passed in heat exchange relation with the main process stream.

5. The. process as claimed, inclaim 1 in which the main .process stream is originally supplied at a low pressurenaniwhich includes the. step of compressing the References Cited in the file of this patent UNITED STATES PATENTS 2,509,034 Claitor et al. Oct. 4, 1948 2,522,787 Hughes Sept. 19, 1950 2,550,886 Thompson May 1, 1951 2,696,088 Twomey Dec. 7, 1954 2,760,356 Sixsmith Aug. 28, 1956 2,764,877 Kohler Oct. 2, 1956 2,783,624 Morrison Mar. 5, 1957 2,896,414 Tung July 28, 1959 

